Basic SCSI FAQ

This FAQ is called a basic SCSI FAQ for good reason - it covers the more common types of SCSI connectors and the fundamentals of the SCSI protocol, and takes you through how to set up and troubleshoot a simple SCSI chain.

There is a lot, lot more to SCSI than this, and our general advice would be not to use SCSI if you have a viable alternative since their are a lot of unpredictable pitfalls. The best advice, if you are considering a SCSI solution that is more complex than is covered here, is to give us a call on 01223 833 412 or send us an email, and one of our staff can give you advice.

SCSI, pronounced "scuzzy", stands for Small Computer Systems Interface. SCSI has been used on all desktop Mac models up until the beige G3, and all laptop Mac models up until the Lombard PowerBook G3.

SCSI is used to describe three things: the method used to transfer data, the connectors on the ends of the cables, and the speed of the device that is hooked up to the computer.

All SCSI connectors have male and female ends, the end with pins being male and the end with holes being female.

This connector is known as DB-25, SCSI One and 25 way SCSI. It has 25 pins and is held in place by two screws. The female connector is on the back of a desktop Mac.

This connector is called Centronics-50, Low Density 50, Wide 50 or 50 pin Centronics. It is held in place by two wires, which snap into the male connector. Many scanners, CDRW drives, and hard drives have this type of connection.

This is the SCSI connector on pre Lombard PowerBooks. It is called HDI-30, HDI stands for High Density Interconnect, and has either 29 or 30 pins. Using the 30th pin enables your PowerBook to be used as an external hard drive. However, most devices will have 29 pins.

This connector is mostly found on fast SCSI devices. It is called Mini DB50, High-density 50 pin, 50 pin Micro D or SCSI Two. It has 50 tightly packed pins. This type of SCSI is used mainly on RAID, higher specification scanners and faster hard drives.

The above image is a high-density 68-pin connector, or High Density DB68 also known as SCSI Three. It is used for extremely fast Ultra Wide SCSI and other solutions. SCSI Three is used on modern SCSI devices. NB: There are a variety of 68-pin SCSI types around today, including LVD (Low Voltage Differential). The ins and outs of LVD SCSI is outside the scope of this FAQ, if you are looking at a solution including an LVD device you really need to examine this area in detail. If you would like any advice on SCSI connectivity, please give us a call on 01223 833 412 or send us an email.

Connecting, maintaining, and troubleshooting SCSI chains can be difficult. But, don't be discouraged - we will now look at how to set up a basic SCSI chain.

For this example we will imagine we have a PowerBook 5300, which has an HDI-30 SCSI connector on it, an Iomega 250MB Zip Drive, with two 25 way SCSI connectors, a EZQuest 24x10x40 CDRW with two C50 connectors, and a fast Hard Drive with two 68 Pin SCSI connections.

Firstly, we turn everything off. It is easiest to sort out the hardware and then worry about the software later.

Secondly, we use a 25way SCSI to HDI-30 cable to connect the PowerBook to the ZIP drive. On a desktop Mac, use the DB25 to DB25 cable that came with the ZIP drive.

Thirdly, we use a DB25 to 50 way Centronics cable to connect the ZIP drive to the CD-RW drive.

Lastly, we use a 50 way Centronics to High Density 68 pin cable to connect the Hard Drive to the CD writer.

Termination

A SCSI chain requires a device called a terminator to be at the beginning and end of a SCSI chain. This prevents signals from "bouncing" back from the unused connectors and interfering with the data. Some SCSI devices come with built-in terminators, which you switch on to activate, while others come with built-in terminators that detect whether or not you need a terminator. Scanners usually come with a separate plug that is just the end of a cable. This type of terminator is called a passive terminator, because it terminates, no matter what the conditions. Active terminators have a circuit that decides whether termination is needed. Macs have built-in passive termination on their end of the SCSI chain.

Since the Hard Drive is on the end of the SCSI chain, it needs a terminator. It will have come with either a passive terminator or a termination switch on the drive itself. Make sure to either have the passive one-ended terminator on the empty port on the scanner, or to have the termination switch set to "on."

SCSI IDs

The built-in SCSI on Macs can support up to 7 devices on its chain. Every device has to have its own, unique ID number in order for the computer to address it properly. If two devices have the same ID number the computer will try to put the data on to both! Remember, Computers are essentially stupid! That is why there's a problem. Each device is different, and so the computer must talk to each one differently.

You can set most SCSI devices to IDs between 0 and 7. IDs 0 and 7, however, are reserved for the Mac itself. So, you are limited to 1, 2, 3, 4, 5, and 6. ID 3 is usually your CD-ROM drive (even if they aren't SCSI drives). ID 5 can also cause problems on PCI Power Macintoshes. So unless you have 4 other devices, avoid 3 and 5 as well.

Setting the SCSI IDs. The zip drive has a switch on the back, next to the termination switch. It has two options, 5 and 6. OK, lets take a glance at the rest... The CDRW has a rotating dial, letting me set options 1 to 7. The Hard drive also has a dial with 1 through 7. Since I want the CDRW to activate first I am going to set that to 1. The Hard drive will be set at 4. This leaves 2, 5, and 6 open for the Zip drive, which itself is limited to 5 and 6. Since SCSI ID 5 has in the past had problems with PCI Macintoshes, I will set it to 6. Now with everything plugged in we are able to turn the machine on.

Now, general rules of SCSI

Termination - Always use termination. If you don't, and it works, fine. Use termination, after the first device (the Mac) and after the last device on the chain. If it doesn't work, try one in the centre of the chain.

SCSI IDs - Never have duplicates. As stated above, do not use ID 0, 3, or 7, because your Mac reserves those. Stick to 1, 2, 4, 5, and 6. On PCI Power Macintoshes, try to avoid ID 5 if possible. It has been linked to a number of problems.

Cable Condition - If your SCSI chain doesn't work, one of the most common problems with it is old, frayed, or defective cables. Always try to isolate the problem to determine if it is the device or the cable, and always use shielded, high quality SCSI cables.

Cable Length - Always try to keep SCSI cables as short as possible. With PowerBooks, it is best to keep cable length very short. If possible, keep it under 3 feet.

The first step: unplug all SCSI devices on the chain. Let the Mac try to start up. If it still fails, with the same symptoms, even after holding down the shift key during start-up, it is not a SCSI problem.

If the Mac works perfectly, then it's SCSI. Pick one device, attach it to the Mac, with any SCSI ID but 0, 3, 5, and 7, terminate it on the end not attached to the Mac, and try it out. Keep doing this, one device at a time, until you find the device that doesn't work. If they all work by themselves, it is a SCSI chain problem, and not a faulty device. Pick 2 devices and hook them both up. Make sure the IDs don't conflict and aren't 0, 3, 5, or 7. Check to make sure that the device furthest from the Mac is properly terminated, and the other device is not terminated. Turn on the Mac. If it doesn't work, try making the first device the last, and vice versa, making sure to change the termination along the way. Turn on the Mac...

Basically, keep going until you have isolated the problem down to one drive, in one type of configuration. From there, check the manuals for the problem device. Call the company's tech support.

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